xfsprogs: fix and clean up installed headers V2

[Christoph Hellwig <hch@xxxxxx>]

This series ensures we only install the minimum required headers, and
especially no autoconf-generated header that might differ for different
targets in multiarch setups.

Note that I'd like to see this in the first xfsprogs release that
includes the libxfs infrastructure changes, as it fixes regressions
in the amount of installed internals in those.

Changes since V1:
  - dropped a cleanup patch Dave didn't like
  - dropped two more cleanup patches that aren't really related